1. Field of the Invention
The present invention generally relates to oscillators, and more particularly, to an oscillator suitable for radio-frequency (RF) circuit.
2. Description of the Related Art
Conventionally, various types of oscillators such as a local oscillator for FM tuners, a crystal oscillator, and a voltage-controlled oscillator are used. A Colpittz oscillator and a Hartley oscillator are known as LC oscillators. The LC oscillator employs a resonance circuit by the combination of an inductor L and a capacitor C. This resonance circuit is capable of generating an oscillation signal over a wide frequency range. The oscillation frequency may be continuously varied by changing the values of the inductor L and capacitor C. However, the oscillation frequency of the LC oscillator fluctuates due to various factors.
For example, a clap oscillator is known as an improved version of the Colpittz oscillator, and has an improved temperature characteristic. The clap oscillator can cover a wide range of a low frequency to VHF (Very High Frequency).
FIG. 1 is a circuit diagram of a conventional circuit configuration of the clap oscillator. Referring to FIG. 1, the clap oscillator has a resonance circuit 1 and a feedback circuit 20. The resonance circuit 1 mainly determines the oscillation frequency, and the feedback circuit 20 mainly determines the oscillating condition. The feedback circuit 20 is made up of an oscillation transistor 21, which is an npn transistor, and feedback capacitors C1 and C2. The feedback capacitor C1 is connected between the base and emitter of the transistor 21, and the feedback capacitor C2 is connected between the emitter and collector thereof. The base of the transistor 21 is connected to the resonance circuit 1. More particularly, the base of the transistor 21 is connected to a main resonance element circuit 11 of the resonance circuit 1 via a clap capacitor C3. The main resonance element circuit 11 may have a self-resonance or series/parallel resonance configuration, which may be realized by a capacitor, an inductor, a diode or a piezoelectric element. The main resonance element circuit 11 is adjusted so that a resonance frequency, which depends on the target application, can be obtained.
The clap capacitor 3 is provided in order to make “coarse” coupling between the main resonance element circuit 11 and the feedback circuit 20. A frequency control terminal 12 is connected to a node at which the main resonance element circuit 11 and the clap capacitor C3 are connected. The frequency control terminal 12 is not an essential element of the resonance circuit 1, and may be omitted. A frequency control voltage applied to the frequency control terminal 12 controls a variable capacitance element in the main resonance element circuit 11. The variable capacitance element may, for example, be a variable capacitance diode. By changing the voltage applied across the variable capacitance diode via the frequency control terminal 12, the oscillation circuit functions as a voltage-controlled oscillator.
Nowadays, the frequency range used in the RF devices is going up as it can be seen from the technical trend in cellular phones or the like, and the individual capacitive elements used in the circuits have reduced values. Generally, there is an increased dispersion of capacitance value as the elements have reduced capacitance values. A serious problem of instability of the circuit operation may arise from an increased dispersion of capacitance. It is therefore required to reduce deviations from the target capacitance values. Particularly, the clap oscillator shown in FIG. 1 employs the main resonance element circuit 11 having a small capacitance value and the feedback capacitors C1 and C2 respectively having small capacitance values. In the clap oscillator, in order to avoid influence on the resonance circuit 1 due to the floating capacitance of the transistor 21 and variation of the load driven by the oscillator, the oscillator is frequently designed to satisfy a condition that the capacitance of the clap capacitor C3 is less than that of the feedback capacitor C1 or C2. Thus, the clap capacitor C3 should have an extreme small capacitance value.
In order to stabilize the oscillation of the clap oscillator with the clap capacitor C3 having an extremely small capacitance value, it is essential to realize reduced dispersion of the capacitance thereof. However, in practice, the floating capacitance of the transistor 21 and variation in the capacitance of the feedback capacitor C1 or C2 may be changed due to variation in the power supply voltage and the ambient temperature. In the case where the main resonance element circuit 11 includes an element that changes the impedance of the resonator in response to an external signal, such as a variable capacitance diode or a crystal vibrator, the tolerable range of the capacitance of the clap capacitor 3 in which the stable circuit operation can be secured can be determined with respect to change of the impedance of the resonance circuit 1 and change of the impedance of the transistor 21. However, there is a limit on reduction of dispersion of capacitance because of an extremely small capacitance of the clap capacitor C3, and such reduction may sometimes be inappropriate in terms of mass productivity.
More particularly, in case where the ratio between the impedance of the main resonance element circuit 11 that is changed by the frequency control voltage applied to the frequency control terminal 12 and the impedance of the clap capacitor C3 is always constant, the impedance of the resonance circuit 1 viewed from the feedback circuit 20 will have only a small change. However, in practice, the ratio is not always constant due to dispersion of impedance of the main resonance element circuit 11 and that of the clap capacitor C3, and the impedance viewed from the feedback circuit 20 varies. Thus, the whole oscillator has a characteristic described by B1 or B2 shown in FIG. 2, in which the horizontal axis denotes the frequency control voltage applied to the frequency control terminal 12, and the vertical axis denotes the C/N (Carrier-to-Noise ratio). As shown in FIG. 2, the C/N ratio changes greatly as the function of the frequency control voltage (that is, the frequency). Further, there is dispersion of the characteristic so that some oscillators have the C/N characteristic B1 or similar thereto, and some oscillators have the C/N characteristic B2 or similar thereto.